Remove shared_preload_libraries description from document.

Remove shared_preload_libraries description from document.

Previously the document explained that shared_preload_libraries must
be set to pg_bigm when using pg_bigm. This description is not true.
pg_bigm can work fine even if shared_preload_libraries is not set.
So this patch removes any descriptions about shared_preload_libraries
from the document.

Per discussion, we decided not to back-patch this to old versions.
Because this change might confuse the existing pg_bigm users who have
already configured shared_preload_libraries. Even if it's configured,
it's harmless, so there is no strong reason why such existing users
should drop that setting from their postgresql.conf soon.